Patent GB-1000874-A (GB)
Title: Improvements in or relating to apparatus producing interference test data for measuring and control instruments
Inventor: SCHUCH KURT
IPC: G01B9/02
Filed: 1961-11-03
Abstract: 1,000,874. Selective signalling. CARL ZEISS JENA VEB. Nov. 3, 1961, No. 39462/61. Heading G4H. [Also in Divisions B3 and G2] In order to extend the measuring range, an interferometer is provided with two reference beams which are used alternately as the length of the measuring beam is continuously varied. The reference beam in use remains of fixed length as the length of the measuring beam increases, and, when the path difference becomes too great to be usable, the other reference beam is switched in, the length of this beam having been altered while the first reference beam was is use by an amount not greater than twice the usuable path difference between the measuring and reference beams. As shown in Fig. 1, parallel light from a source 11 is split at 16 into a measuring beam 17 and a beam 18 which is split at 23 to form two reference beams 18<SP>1</SP>, 18<SP>11</SP>. The three beams are reflected back by prisms 20, 24 and 26 to a prism 22 where they are re-combined and focused by a lens 32. As shown, a diaphragm 31 stops out the reference beam 18<SP>11</SP> so that only beams 17 and 18<SP>1</SP> are re-combined, to form interference fringes which move across two apertures in a diaphragm 33 as the prism 20 is moved in the direction A to increase the length of the measuring beam 17. A prism 34 behind diaphragm 33 leads the light alternately to photo-cells 35, 36 in circuit with an electronic apparatus 37 which counts the fringes and indicates their direction of movement. As prism 20 moves, prism 24 remains fixed, but prism 26 moves through a distance not greater than twice the usable path difference of the light employed to a new position 261. When the path difference between beams 17 and 18<SP>1</SP> becomes so great that the fringes are no longer well-defined, diaphragm 30 stops out the beam 18<SP>1</SP> and diaphragm 31 opens so that for further movement of prism 20 in direction A the reference beam 18<SP>11</SP> is utilized for fringe formation. Prism 24 may now be moved again to initiate a further cycle. At the time of changeover the path difference between beams 17 and 18<SP>1</SP> is arranged to be slightly greater than that between beams 17 and 18<SP>11</SP> so that the fringes produced by these two beams are of greater contrast than the fringes formed at the same time by beams 17, 18<SP>1</SP>. In other forms, the reference beams 18<SP>1</SP>, 18<SP>11</SP> are combined with different split portions of the measuring beam so that two sets of fringes are produced, the fringes in each set being counted by separate photocells and electronic equipment which are coupled by an electronic switch. A large increase in a reference beam length with only a small movement of the prism may be obtained by sliding the prism 128, Fig. 6, between parallel reflectors 126, 127. The interferometer may be used to measure the movement of a slide 93, Figs. 4, 5, across the bed 90 of a machine tool. The slide 93 carries a prism 120 for returning the measuring beam, prisms 121, 122 for the two alternatively used reference beams being carried by slides 98, 99. These slides have guides 106, 107 in which may slide tappets 104, 105 fast with radial arms 102, 103 integral with gearwheels 100, 101, which are rotatably mounted about axes X-X and Y-Y in the slide 93 and mesh with racks 94, 95 on the fixed machine bed 90. The two arms 102, 103 are so arranged that one tappet 104 is engaged in its slide 106 whereas the other tappet 105 is not engaged with its slide 107. When slide 93 is moved in direction B the gears rotate in opposite directions and slide 98 moves in the same direction B while slide 99 remains fixed until its guide 107 is engaged by the tappet 105. Slide 99 then moves and slide 98 remains fixed. Each slide 98, 99 is associated with clamps having jaws 116, 117 and 118, 119 for co-operation with supports 96, 97 integral with the machine bed 90, the clamps being operated by introducing air into a chamber, such as 112, through a duct 110.